C - threads and processes - prevent zombies - c

Question

I'm implementing a function which receives a parsed-to-array command lines ("./waiter 20 &" will be parsed, and the function will receive the array
{"./waiter","20","&"} for example).
If the last argument is &, the process should run in the background.
To prevent zombies, I need to use a new thread that will wait for the child process.
The attached code is my working program, and all my efforts to add a new thread that will wait for the child process failed.
Anyone can guide me?
Attaching the code, with some of the leftovers of my unsuccessful tries.
(the function is process_arglist)
update: after a lot of tries using what suggested here, its still failing and im not sure why. updated code attached.
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <errno.h>
#include <string.h>
#include <pthread.h>
void func(void* ptr) {
pid_t* mypid = (pid_t*)ptr;
waitpid(*mypid);
pthread_detach(pthread_self());
}
int process_arglist(int count, char** arglist){
int isBackground = 0;
pid_t pid;
int status;
char** parsedList;
if (strcmp(arglist[count-1],"&") == 0) {
printf("###we are in the & situation\n");
parsedList = (char**)malloc((count-1)*sizeof(char*));
if (parsedList == NULL) {
printf( "Error: malloc failed - %s\n", strerror(errno));
}
int i;
for (i=0;i<count-1;i++){
parsedList[i] = arglist[i];
}
/*printf("parsed list:\n");
for (i=0;i<count-1;i++) {
printf(" %d: %s\n", i,parsedList[i]);
}*/
if ((pid = fork()) < 0) { /* fork a child process */
printf( "Error: fork failed");
exit(0);
} else if (pid == 0) { /* for the child process: */
if (execvp(*parsedList,parsedList) < 0) { /* execute the command */
printf( "Error: execvp failed - %s\n", strerror(errno));
exit(0);
}
} else {
pthread_t thread;
pthread_create(&thread, NULL, (void*) &func, (void*) &pid);
}
} else {
if ((pid = fork()) < 0) { /* fork a child process */
printf( "Error: forking child process failed - %s\n", strerror(errno));
exit(0);
}
else if (pid == 0) { /* for the child process: */
if (execvp(*arglist,arglist) < 0) { /* execute the command */
printf( "Error: execvp failed - %s\n", strerror(errno));
exit(0);
}
}
else { /* for the parent: */
while (waitpid(&status) != pid); /* wait for completion */
}
}
}

Answer 1

First, switch from calling wait to calling waitpid. Otherwise, if you have more than one thread waiting, they'll steal each other's notifications.
Second, break the call to waitpid into its own function that takes the PID to wait for as a parameter. Cast it through void *, since that's what's used for thread parameters.
Third, change the call to the function to a call to pthread_create, casting the PID to wait for to a void * to pass to the newly-created thread.
Lastly, have the thread detach itself, since there won't be anything waiting for the thread to terminate.